Since I want to know which is right...
Option a) Ross's (as in "This is Ross's car")
Option b) Ross' (as in "This is Ross' car")
Option a) Mistress's (as in "Mistress's dress")
Option b) Mistress' (as in "Mistress' dress")
Option a) Bus's (as in "Bus's headlights swept the road")
Option b) Bus' (as in "Bus' headlights swept the road")
I would - and have - used option b) on all those, and am annoyed whenever I see the 's following the final s (I know there is probably a fancy word for that, but I am not that interested in the matter, other than to hear what is the right way to put it). I am currently reading a book with loads of "unnecessary" s, and I want to know how it is REALLY done.